The goal of vegetable breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to diseases, insects or other pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, enhanced growth rate and improved fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same genotype. A plant cross-pollinates if pollen comes to it from a flower of a different genotype.
Plants that have been self-pollinated and selected for a uniform type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny of homozygous plants. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. The extent of heterozygosity in the hybrid is a function of the genetic distance between the parents. Conversely, a cross of two plants each heterozygous at a number of loci produces a segregating population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
Tomato cultivars may be grouped by maturity, i.e. the time required from planting the seed to the stage where fruit harvest can occur. Standard maturity classifications include ‘early’, ‘midseason’ or late-maturing'. Another classification for tomatoes is the developmental timing of fruit set. ‘Determinant’ plants grow foliage, then transition into a reproductive phase of flower setting, pollination and fruit development. Consequently, determinant cultivars have a large proportion of the fruit ripen within a short time frame. Growers that harvest only once in a season favor determinant type cultivars. In contrast, ‘indeterminate’ types grow foliage, then enter a long phase where flower and fruit development proceed along with new foliar growth. Growers that harvest the same plants multiple times favor indeterminate type cultivars. In response to more recent consumer demands for dietary diversity, tomato breeders have developed a wider range of colors. In addition to expanding the range of red colored fruits, there are cultivars that produce fruits that are creamy white, lime green, yellow, green, golden, orange and purple. Additionally, there are multi-colored varieties exemplified by mainly red fruited varieties with green shoulders, and both striped- and variegated-colored fruit.
The fruits of tomato plants which are more suitable for processing are generally red colored and have pink to red/crimson fruit flesh.